A flashback to when Leonard first moved in with Sheldon and they signed the roommate agreement.
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Before there was Leonard, there was an even angrier roommate. Angry enough to write "Die Sheldon Die."
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A flashback to the nerds when they first met playing Gamecube together.
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Sheldon runs into a character played by Mayim Bialik on the 5/17/10 episode of The Big Bang Theory. Might she be the one for him?
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You gotta love this guest-starring appearance on The Big Bang Theory: that's former sitcom star Mayim Bialik!
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This is a scene from the 5/17/10 episode of The Big Bang Theory. Yes, that's Mayim Bialik guest-starring in the sitcom.
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Judy Greer guest starred on The Big Bang Theory as Dr. Plimpton, a famous physicist that Sheldon invites to spend the weekend at the place.
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Penny returns Sheldon to Leonard after their trip to Disney Land. The couple is sharing custody over the nerd.
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The Big Bang Theory Season 3 Quotes

Penny: I give up, he's impossible
Sheldon: I can't be impossible, I exist. I believe what you meant to say is, "I give up, he's improbable."

Yes, in 1917 when Albert Einstein established the theoretic foundation for the laser in his paper "Zur Quantentheorie der Strahlung," his fondest hope was that the resultant device be bitchin'.

Sheldon